1. Field of the Invention
The present invention relates to a method and apparatus for preheating glass batch before being inputted to a glass melting furnace.
2. Description of the Related Art
Because a glass melting furnace consumes a large quantity of thermal energy, various energy-saving measures have been put into practice. However, the quantity of heat taken away by waste gas from a melting furnace remains to be at a high level corresponding to more than 20% of the inputted fuel, and thus, as a powerful means for effectively utilizing the waste heat, it has been a big problem for the glass manufacturing industry to put preheating glass batch with waste gas to practical use. In spite of this, the actual situation is that the preheating has not reached the level of practical use. In the following, prior art and the reason that the technology is prevented from being put to practical use are described.
As a conventional method for preheating glass batch, both a direct heating method and an indirect heating method have been proposed. The direct heating method has an advantage that the efficiency of heat transfer is higher and the apparatus can be miniaturized that much more, since waste gas is forced to pass through a glass batch layer and heat is transferred directly. However, since waste gas discharged from a glass melting furnace is contaminated waste gas containing volatile glass component and combustion product of fuel, when these contaminants get in contact to cool glass batch or moisture attached thereto, for example, in case of bottle glass using Glauber's salt as the refining agent, Na.sub.2 SO.sub.4, Na.sub.2 S.sub.2 O.sub.7, NaHSO.sub.4, and sulfuric acid are produced, which contaminate the glass batch to lower the quality of the glass and corrode metal constituting the apparatus. There are many other problems, for example, the necessity of reinforcing a dust collector and larger pressure loss of the waste gas, because of fine powder batch scattered and mixed in the waste gas. On the other hand, according to an indirect heating method, since glass batch is heated by "contaminated waste gas" via heating tubes, there are advantages that problems such as contamination of glass batch by "contaminated waste gas," damage by acid of the apparatus, and increase in the amount of dust can be avoided, and the pressure loss of the waste gas is small. However, on the other hand, there are problems that volatile glass component and combustion product of fuel contained in waste gas from a glass melting furnace attach to the inner face of the heating tubes to lower the efficiency of heat transfer and clogs a path of waste gas. Further, some of the attaching substances are welded to the inner face of the heating tubes at 400.degree. C. or below, and it is quite difficult to remove the attaching substances during operation of the apparatus.
As mentioned in the above, in a conventional technology of preheating glass batch, both a direct heating method and an indirect heating method adopt introduction of "contaminated waste gas" itself containing a large amount of contaminants into a batch preheater as the heat source. This makes a glass melting system with a batch preheater installed therein to have difficult problems given in the above. More specifically, in a conventional method in which "contaminated waste gas" is directly introduced into a batch preheater, the advantage that energy is saved by collecting waste heat is overwhelmed by the disadvantages that are caused with respect to, for example, the maintenance and operation of the equipment and the quality of the glass, which loses appeal of preheating glass batch. In conclusion, it can be said that to care conventionally that "contaminated waste gas" itself containing a large amount of contaminants is directly introduced into a batch preheater prevents preheating of glass batch from being put to practical use.
Further, nowadays there is a tendency that, from a standpoint of an antipolution measure, a glass melting furnace by pure oxygen combustion is adopted. In this case, since, different from a conventional furnace, preheating of combustion air can not be used as a means for recovering waste heat, the actual situation is that thermal energy of hot waste gas is discharged to the atmosphere without being effectively utilized. Application to preheating glass batch of waste heat of an oxygen combustion glass melting furnace is now described in the following.
When glass batch is preheated utilizing waste heat from a glass melting furnace, the waste gas to be introduced into a batch preheater is required to be 600.degree. C. or below so that glass cullet is not fused and solidified. In case of a conventional air combustion glass melting furnace, since a regenerator or a recuperater (such as heat exchanger made of metal) for preheating combustion air is located for recovering waste heat, the temperature of the waste gas is lowered to about 500.degree. C. in case of a regenerator and about 600.degree. C. in case of a recuperater, and accordingly, the waste gas can be introduced into a batch preheater with the temperature unchanged. However, in case of an oxygen combustion grass melting furnace, since there is no need to pretreat combustion air, recovery of waste heat must be attained by some other process. However, since an appropriate waste heat recovering apparatus to take the place of preheating has not yet been developed, the actual situation is that waste gas as hot as 1450.degree. C. or above is discarded.
In case glass batch is preheated in an oxygen combustion glass melting furnace system, the waste gas to be introduced into a batch preheater is required to be cooled anyhow to 600.degree. C. or below. However, cooling by dilution with ordinary temperature air leads to increase of the amount of the waste gas, and thus the meaning of adopting oxygen combustion from a standpoint of an antipolution measure is lost. An apparatus for preheating glass batch utilizing waste heat from an oxygen combustion glass melting furnace has not yet been put to practical use.
In the above context, with regard to glass melting furnace including an oxygen combustion glass melting furnace, to put a glass batch preheater to practical use is to materialize an effective means contributing not only to reduction of consumption of fuel used to melt glass and to reduction of Nox but also to reduction of carbon dioxide production due to combustion. In view of the above the present invention provides a new glass batch preheater, developed for the purpose of application to an oxygen combustion glass melting furnace which is expected to prevail in the future, not to mention a conventional regenerative or recuperative glass melting furnace.
Accordingly, a problem to be solved by the present invention is to develop a method and apparatus for preheating glass batch satisfying the following items:
(1) Without adopting a conventional method of introducing "contaminated waste gas" which contaminates glass batch and which makes dust attach and deposit to the inner face of heating tubes directly into a batch preheater, a clean heat medium gas with high heat transfer efficiency heated by hot "contaminated waste gas" shall be used as an alternative heat source, and the apparatus shall not adversely influence the glass quality, nor shall it require maintenance such as dust cleaning; PA1 (2) The method shall reheat heat medium gas by endlessly circulating it in order to improve heat recovery from waste gas and to make the heat medium gas as hot as possible; PA1 (3) Since glass batch typically contains 3-5% of moisture end water vapor produced by heating batch recondenses in a specific batch layer in a low temperature range causing solidification in mass of grass batch at the recondensing point resulting in clogging a channel of gravitationally dropping batch, the apparatus shall be constructed such that produced water vapor is promptly discharged. PA1 (4) In order to prevent solidification of batch and to facilitate heat transfer, a heating chamber of a batch preheater shall be constructed such that gravitationally dropping batch layers exhibit gentle dispersion phenomenon; and PA1 (5) Adding the present apparatus shall not substantially influence the equipment, operation, and the like of a conventional glass melting furnace.
In order to attain the above-mentioned object, according to the present invention, as shown in FIGS. 1 to 3, in a method for preheating glass batch utilizing waste heat of hot gas discharged from a glass melting furnace before glass batch is inputted to the furnace, instead of introducing hot "contaminated waste gas" directly into a glass batch preheater, carbon dioxide or water vapor having heat radiation intensity in the infrared ray wavelength range is used as the heat medium. The heat medium is heated by a heat exchanger for recovering waste heat (hereinafter referred to as heat exchanger for heating heat medium gas or simply as heat exchanger) located in a waste gas flue of the furnace and is introduced via an inline duct of a circulating circuit into a glass batch preheater to preheat glass batch. The heat medium gas remains 200.degree. C. or above even after exiting the batch preheater, and returns via a blower provided in the middle of an outline duct of the circulating circuit to the heat exchanger to be heated to raise its temperature. Hotter heat medium gas could be successfully obtained by this method in which the heat medium gas is endlessly circulated to be reheated. By using the hotter heat medium gas as the heat source for preheating glass batch, the above-mentioned various problems caused according to a conventional method, that is, a method in which "contaminated waste gas" containing volatile glass component and combustion product of fuel is directly introduced into a batch preheater could be solved.
Further, by using heat radiative (heat absorbent) gas such as carbon dioxide or water vapor as the heat medium for preheating glass batch, when heat is absorbed from waste gas by the heat medium gas in a heat exchanger or when heat is emitted to batch in a glass batch preheater, not only conduction and convection, but also radiation contributes to heat transfer (in case air is used, heat is not transferred by radiation), and thus, rate of heat transfer is increased, and as a result, the equipment could be miniaturized.
It is to be noted that, though, other than carbon dioxide and water vapor, SO.sub.2, CO, NH.sub.3, Hcl, hydrocarbon of various kinds, and the like are gases having heat radiation (absorption) intensity in the infrared ray wavelength range, the most suitable heat medium gases for an apparatus of the present invention are carbon dioxide and water vapor, and either one is singly used or both are mixedly used. Since air (N.sub.2 and O.sub.2) does not radiate (absorb) heat and is therefore inappropriate for heat medium gas, and since lowered partial pressure of heat medium gas leads to weaker heat radiation intensity, the equipment is constructed such that air does not come in the circulating circuit.
Further, in order to prevent water vapor produced by heating wet batch from recondensing in a specific batch layer in a low temperature range causing solidification in mass of batch at the recondensing point resulting in clogging a channel of batch, the bottom of each heating tube is constructed to be flat or concave such that a cavity which is U-shaped in cross section is formed in a batch layer right under each heating tube by angle of repose of gravitationally dropping batch to collect water vapor in the cavity and to promptly suck and discharge it to the outside via an exhaust pipe.
Each step of the heating tubes in the glass batch preheater is preferably about 650 mm or less in height in an upper zone (drying zone) of a heating chamber, and about 650 mm or more in height in a lower zone (heating zone). The total number of the heating tubes and the number of steps of the heating tubes are set depending on various conditions such as the quantity of preheat of glass batch, the quantity of attached moisture, and the kind and composition of batch. Particularly, with respect to the upper zone (drying zone) of the heating chamber, the number and arrangement of the heating tubes are required to be decided with complete vaporization of moisture in glass batch (typically 3-5% of moisture is contained) and rising temperature of batch itself to the boiling point of water or above being as essential conditions. Since, depending on the composition of glass batch, glass batch sometimes separates and vaporizes constitution water at 500.degree. C. or above and is sometimes decomposed to produce gas, for the purpose of discharge them to the outside, a gas exhaust pipe is attached at the bottom of each heating tube in the lower zone similarly to the upper zone. The heating tubes in the upper zone and in the lower zone are similar in shape except for the height.
Further, in order to make gravitationally dropping batch layers exhibit gentle dispersion phenomenon to prevent solidification of batch and to facilitate heat transfer, the heating tubes in the heating chamber are arranged to be staggered step by step in the direction of the height to make the gravitationally dropping batch layers repeat dispersion at the apices of the heating tubes of each step. As a result, the particles of batch did not grow in mass to clog a channel of batch. Further, the dispersion of batch led to equalization of temperature of batch resulting in higher efficiency of heat transfer.
Still further, according to the present apparatus, since waste gas from the glass melting furnace need not round to the glass batch preheater to be introduced as the heat source, adding the present apparatus does not substantially influence the equipment, operation, and the like of a conventional glass melting furnace. If forced to mention, since a heat exchanger for heating heat medium gas is located in a waste gas flue of the furnace, the pressure loss of waste gas through it becomes a burden of an existing exhauster. However, since a heat exchanger operable within the surplus capacity of the exhauster is selected, the existing furnace equipment is not substantially influenced.